Project Summary/Abstract. The objectives of this project are to determine how prions enter the body, infect lymphoid tissue and travel via peripheral nerves to gain access to the central nervous system where they cause disease. These objectives will be met using a natural model of infection that has relevance to the horizontal spread of prion disease between animals, such as chronic wasting disease in deer and elk, and scrapie in sheep and goats. For these studies hamsters will be inoculated extranasally with prions, which they immediately inhale into their nasal cavity. This route of inoculation is likely to be relevant to the natural spread of prion diseases because a study published this year demonstrated that sheep can be infected with scrapie via intranasal inoculation. We have recently shown that this route is 10-100 times more efficient in causing infection than oral inoculation, so a lower dose will cause disease if it enters the body via this route. We will study the early events in the transport of prions in the nasal cavity by collecting tissue from animals at various time points after inoculation and establishing the time course of transepithelial transport, and the specific cells that mediate this transport, using immunohistochemistry. We will determine the mechanisms for the relative efficiency of this route of infection by measuring the amount of normal prion protein present in the lymphoid tissue, the density of innervation, and the ability of the lymphoid tissue to convert to the infectious isoform of the prion protein. The mechanism of axonal transport via peripheral nerves will be studied by removing segments of nerves known to transport prions following extranasal inoculation at different times after inoculation, and determining the pattern of prion transportation using immunohistochemistry. The role of the peripheral ganglia in neuroinvasion will be studied by measuring the amount of infectivity in the ganglia at regular intervals after inoculation. In addition to identifying early events in prion transport, replication and neuroinvasion in a natural model of infection, the information obtained from these studies will increase our general understanding of the pathogenesis of prion diseases, and therefore will have relevance to our understanding of bovine spongiform encephalopathy (mad cow disease) and variant Creutzfeldt-Jakob disease in humans.